Oops, I seem to be chasing Alan around... my apologies.
As simple as it sounds, I find the easiest way to find a centre (whether centre channel or setting a rough pan in a stereo mix) is to close my eyes, shake my head a bit to disorient myself (not like a Stevie Wonder head bob!) and point to where I hear the mix coming from, open your eyes and see how far off you are and adjust. It works for me because I've got nearly identical hearing in both ears (despite spending about a tenth of my adult life wearing a left-ear-only ClearCom) - your mileage may vary.
Bren R.
